A Local Wave Activity Interpretation of the Large Scale Drivers Behind the Projected Drying of the Mediterranean

The projected drying of the Mediterranean basin is a robust signal of future climate change. Dynamically, this is manifested as a large anticyclonic anomaly covering the region, alongside a decrease in cyclonic activity. Various processes have been previously proposed as drivers of this trend, both thermodynamical (decreased land-sea temperature gradient) and dynamical (intermediate-scale stationary wave response, poleward shift of the jet stream). Several elements of the North Atlantic large-scale circulation are known to affect Mediterranean cyclonic activity (extratropical storms, jet stream position, weather regimes), individually and through mutual interactions. However, their contribution to the overall drying remains an open question.

In this work, we use the framework of Finite Amplitude Local Wave Activity (FALWA; Huang & Nakamura, 2016) to deconstruct the role of the North Atlantic circulation in the projected changes downstream. FALWA is a diagnostic that keeps track of the wave activity ”stored” within circulation undulations, relative to a zonalized flow. It obeys an exact conservation relation; thus, its local rate of change is either due to a flux convergence, or to non conservative source-sink terms. This allows for a closed mechanistic budget analysis of the response, differentiating between horizontal advection by the mean flow, barotropic and baroclinic processes, and diabatic forcing.

We analyse this budget in a 10 member CMIP6 ensemble and investigate the multi-model mean circulation response and ensemble spread over the North Atlantic and the Mediterranean basin. Preliminary results show a prominent baroclinic contribution over the eastern North Atlantic combined with enhanced advection of upper tropospheric potential vorticity towards Europe. Both elements imply that the shifting of the North Atlantic storm track plays a role in the projected drying trend of the Mediterranean.